I. Field of the Invention
This invention relates to a process for producing films made up of two different metal oxides in which a first metal oxide predominates at one side of the film and a second metal oxide predominates at the other side of the film (such films being referred to hereinafter as bilayer oxide films). The invention also relates to bilayer oxide films produced by the method and to devices incorporating such films.
II. Description of the Prior Art
Metal oxides have a variety of useful physical and chemical properties, such as inertness, stability, abrasion resistance, resistance to high temperatures, etc. This makes metal oxides particularly useful as coatings for other materials or in other applications in which they are required to be in the form of supported or unsupported thin films. We have also found it particularly useful for certain applications to provide bilayer oxide films which make use of the different properties of two different metal oxides.
Metal oxide films are often produced by vapour deposition techniques but these methods are not always entirely suitable. For example, these methods involve high temperatures due to the high melting point (for evaporation) and high heat of condensation (for sputtering) of these oxide materials. Consequently heat sensitive substrates can be damaged or destroyed. Vapour deposition techniques also usually involve the use of high vacuums which may cause out-gassing from certain materials, such as paper, making the methods unsuited for the coating of such substrate materials. A further disadvantage is that it is expensive using the known techniques to produce bilayer films, i.e. those in which a first metal oxide predominates at one side of the film and a second metal oxide predominates at the other side of the film.
It is accordingly an object of the present invention to provide a process for producing bilayer metal oxide films which overcomes these disadvantages and makes it possible to produce films of this type as coatings on a variety of substrates or as unsupported films.
One possible way of producing bilayer oxide films is by anodizing a structure comprising a layer of a metal which can be anodized to consumption (e.g. aluminum) deposited on a metal which at least forms a barrier anodic film (e.g. a valve metal). However, oxide films formed in this way normally adhere tenaciously to the substrate metal and cannot therefore be transferred to other substrates. This type of procedure is mentioned in Russian Patent No. 817 099 to P. P. Khanzhin et al published on Mar. 30, 1981. In this reference, an aluminum foil is deposited (by rolling or vacuum spraying) onto a substrate made of a valve metal (e.g. titanium) and anodization is carried out until all the aluminum and some of the underlying valve metal is consumed. According to this reference, the anodic layer can easily be separated from the substrate "by a slight mechanical action" with the separation taking place at the interface between the aluminum oxide layer and the valve metal oxide layer. It would therefore seem that a bilayer oxide film is not produced when the film is separated from the substrate. Moreover, attempts to duplicate this procedure have resulted in the formation of anodized films that cannot easily be removed from the substrate.
A similar procedure is disclosed in an article by W. L. Baun entitled "Anodization of evaporated aluminum on Ti-6 wt. % Al-4 wt. % V", Journal of Materials Science, 15 (1980) 2749-2753. Baun tried to adapt the well-known technology for porous anodizing Al, used to prepare the surface for adhesive bonding, to Ti by evaporating a layer of Al onto a Ti substrate and then treating it according to the standard procedures known for Al. The anodic film thus obtained did not have the porous structure associated with similarly prepared films on Al alloys. Rather than a bilayer film, a non-porous mixed Al--Ti oxide structure was formed. Furthermore, the article states that the duplex oxide forms a strong bond to the underlying metal and presumably could not be easily detached.